Orthogonal frequency-division multiplexing (OFDM) is a widely used digital modulation/multiplexing technique. Coherent optical orthogonal frequency-division multiplexing (CO-OFDM) is being considered as a promising technology for future high-speed (e.g., 100-Gb/s per-channel data rate) optical transport systems. In high-speed long-haul optical fiber transmission based on CO-OFDM, large guard-interval (GI) between adjacent OFDM symbols is needed to accommodate fiber chromatic dispersion (CD) induced inter-symbol interference (ISI).
Chromatic dispersion (CD) is a deterministic distortion given by the design of the optical fiber. It leads to a frequency dependence of the optical phase and its effect on transmitted signal scales quadratically with the bandwidth consumption or equivalently the data rate. Therefore the CD tolerances are reduced to 1/16, if the data rate of a signal is increased by a factor of four (4). Up to 2.5 Gb/s data rate optical data transmission is feasible without any compensation of CD even at long haul distances. At 10 Gb/s, the consideration of chromatic dispersion becomes necessary, and dispersion compensating fibers (DCF) are often used. At 40 Gb/s and beyond, even after the application of DCF the residual CD may still be too large for feasible optical communication.
Another transmission impairment experienced in CO-OFDM transmission is Polarization-Mode Dispersion (PMD), which is a stochastic characteristic of optical fiber due to imperfections in production and installation. Pre-1990 fibers exhibit high PMD values well above 0.1 ps/√km which are border line even for 10 Gb/s. Newer fibers have a PMD lower than 0.1 ps/√km, but other optical components in a fiber link such as reconfigurable add/drop multiplexers (ROADMs) may cause substantial PMD. If 40 Gb/s systems are to be operated over the older fiber links or over new fiber links with many ROADMs, PMD may become a significant detriment. PMD can be compensated by optical elements with an inverse transmission characteristics to the fiber. However, due to the statistical nature of PMD with fast variation speeds up to the few kHz range, the realization of optical PMD compensators is challenging. With increases in channel data rate, optical signal is more and more limited by the transmission impairments in optical fiber, such as by CD and PMD.
The insertion of large GI between adjacent OFDM symbols to accommodate large CD leads to substantial reduction of transmission spectral efficiency (SE) when the GI becomes a large fraction of the OFDM symbol length. One obvious way to solve this problem is to proportionally increase the OFDM symbol length so the overhead due to the GI remains low, however, this leads to proportionally large reduction in the frequency spacing between OFDM subcarriers, which in turn leads to unacceptably high requirement on the optical frequency locking between the transmitter laser and the receiver optical local oscillator (OLO). In order to enable high-speed CO-OFDM to be spectrally efficient and tolerant to the frequency offset between the OLO and the transmitter laser in the presence of large fiber dispersion, according to a scheme commonly referred to as “No-Guard-Interval Coherent OFDM”, the GI removed, and blind equalization at the receiver relied on to compensate for fiber dispersion.